Monday, August 7, 2017

Gradients

Gradients have become an essential part of modern NMR spectroscopy. Nearly all probes now have pulsed field gradient capabilities and the vast majority of pulse sequences use gradient pulses. In this post, and those following, I will try and explain what gradients are and how they can be useful.

Sunday, July 2, 2017

Sensitivity vs receiver gain

Recently I have noticed many users recording spectra with low values for the receiver gain. The receiver gain is a scaling factor for the FID signal that ensures spectra are not distorted if the signal is too large, or resolution is lost if the signal is too small. Typically the receiver gain is set automatically by the command "rga", but users should pay attention to the value obtained as it is a good indicator of how successful the experiment will be. To demonstrate this I measured sensitivity at different receiver gain values.

Friday, June 2, 2017

15N HMBC sensitivity

The HMBC experiment is normally used to detect 1H-13C correlations, but it can also be used for 15N. An 15N HMBC experiment may provide structural information that cannot be obtained from a 13C HMBC, and it can also be used to determine the number of nitrogen atoms present in a compound. Recently I was asked, "How does the sensitivity of an 15N HMBC compare with that of a 13C HMBC?", so I decided to try and find out.

Monday, May 8, 2017

Variants of the 1D 13C experiment

1D 13C spectra are often used to confirm compound identity. The simplicity of such spectra enable the number and type of carbon atoms present to be quickly evaluated. 2D 1H-13C correlation spectra, while inherently more sensitive and informative, require more work to interpret and may not show signals from all carbon atoms. Consequently, 1D 13C spectra still have a place in compound characterization. To increase the sensitivity and information content of 1D 13C spectra several variants of the original pulse sequence have been developed. Several of these are discussed below.

Wednesday, April 5, 2017

Single bond correlations in HMBC spectra

HMBC spectra are designed to show correlations between hydrogens and heteronuclei separated by two or more bonds, however, single bond correlations are often present in HMBC spectra as well. Like the multiple bond correlations in HSQC spectra discussed in the previous post, single bond correlations in HMBC spectra can be confusing if you are not expecting them, but once aware that they may be present they are fairly easy to identify.

Wednesday, March 8, 2017

Multiple bond correlations in HSQC spectra

1H-13C HSQC spectra are designed to show correlations between carbons and their directly attached protons. Interpretation of all the correlations observed in a HSQC spectrum is an essential step in characterizing a new molecule. It is not uncommon, however, to observe HSQC correlations over more than one bond. At first these long range correlations can be quite confusing, but if one is aware of the possibility of long range correlations being present, they can actually help the analysis.

Tuesday, February 7, 2017

Assigning a simple compound: ODCB

ODCB (o-dichlorobenzene) is a simple aromatic compound used as a standard for monitoring 1H resolution. In a 1H spectrum it produces just two peaks, due to the symmetry of the molecule, but it is not immediately obvious which nuclei cause which peaks.